Generally, car bodies of rolling stock are produced of light alloys such as aluminum alloys, steel for structural use or stainless steel materials. In the car bodies of rolling stock, the whole body is essentially constructed of one type of material.
A car body produced or a stainless steel material has a long life because the stainless steel is less susceptible to corrosion than an aluminum alloy and steel. A stainless steel body, however, requires a special surface treatment of its outer places when the body surface is to be painted. Therefore, the stainless steel body requires considerable time and labor for painting. A car body built of an aluminum alloy material is light in weight and easy to paint; however, an aluminum alloy is more expensive than stainless steel. Therefore the material cost of aluminum alloy car body is higher than that of stainless steel body and body built of steel for structural use. However, the steel material for structural use is susceptible to corrosion as compared with the stainless steel and heaver than the aluminum alloy material. The car body built of steel for structural use is heavier than the aluminum alloy body and has a shorter service life than the stainless steel body.
Recently various technologies have been proposed for joining different types of materials. Also there has been achieved improvement in the strength of the connecting section of material, thereby enabling the combination of the different types of materials, for example, a light alloy material and a steel material for structural use. In the meantime, there has been an increase in the number of railway cars equipped with control equipment and air-conditioning apparatus on the undersurface of an underframe. In a car body described above, it is imperative to construct the underframe such that the various equipment can be mounted on the undersurface of the underframe as easily as possible. The underframe, therefore, is built of a plurality of extruded shapes of aluminum alloy formed integral with a rail sturdy enough to support the various equipment. The plurality of the extruded shapes of aluminum alloy extend in the longitudinal direction of the car and are arranged in parallel with the direction of width of the car body and joined to one another by welding. A side framing of a stainless steel material or a steel material is used for structural purposes. A car body using an underframe made up of the extruded shapes of an aluminum alloy joined with the side framing is proposed in, for example, Japanese Laid-Open Utility Model No. 60-108584, wherein the entire body of the underframe is composed of extruded shapes of aluminum alloy; however, no consideration is given to the reduction of material costs in fabricating the underframe.
Another example of the car body underframe composed of extruded shapes of aluminum alloy is proposed in, for example, Japanese Laid-Open Patent No. 62-94466, wherein an equipment mounting section is provided separately from sections for other equipment is built as one frame by assembling and joining these sections. Since this underframe is generally constituted of the extruded shapes of aluminum alloy, material costs are increased.